FIG. 1 is a drawing for explaining a connection cable between an inverter, which is a power converter driven by PWM control, and a motor. In FIG. 1, an inverter 1, which is a power converter, has connected thereto a motor 2 via a cable 3. In the inverter 1, a switching operation of semiconductor switching elements (for example, IGBT elements) is controlled through PWM control by a controlling unit not shown to generate three-phase voltages (uvw) varying stepwise from a direct-current power supply having a voltage Vdc, and these voltages are output to the motor 2 via the connection cable 3.
Here, when this connection cable 3 between the inverter 1 and the motor 2 is long, a surge voltage exceeding twice a direct-current bus voltage Vdc may occur at cable-connection ends of the motor 2. That is, the connection cable 3 can be considered as a resonant circuit composed of a wiring inductance and a floating capacitance. When the connection cable 3 is long, the wiring inductance and the floating inductance are both increased, thereby reducing a resonant frequency of the resonant circuit. As a result, before resonance excited at the resonant circuit due to a stepwise change in voltage produced by the inverter 1 is attenuated, the next stepwise change in voltage is applied. Such repeated application increases resonance, thereby causing a surge voltage, which is a voltage higher than usual, at the cable connection ends of the motor 2.
With reference to FIGS. 2A and 3B, details of the surge voltage occurring at the cable connection ends of the motor 2 are described. FIGS. 2A and 3B are drawings that depict line-to-line voltage waveforms at both ends of the connection cable 3 shown in FIG. 1.
FIG. 2A depicts a case where an inverter-end line-to-line voltage Vuv_inv is varied stepwise as Vdc→0→Vdc. At this time, when a pulse width in voltage change coincides with half of a resonant cycle, as shown in FIG. 2B, a motor-end line-to-line voltage Vuv_motor becomes three times as high as the direct-current bus voltage Vdc at maximum.
Also, FIG. 3A depicts a case where the inverter-end line-to-line voltage Vuv_inv is varied stepwise as 0→Vdc→−Vdc→0. At this time, as shown in FIG. 3B, the motor-end line-to-line voltage Vuv_motor becomes four times as high as the direct-current bus voltage Vdc at maximum.
From the description with reference to FIGS. 2A and 3B, it is known that if the pulse width in voltage change is sufficiently large, after resonance occurring due to a stepwise voltage change is attenuated, the next stepwise voltage change is applied, and therefore a surge voltage exceeding twice the direct-current bus voltage Vdc does not occur.
To solve this surge-voltage problem, for example, first and second patent documents disclose a technology of monitoring a firing pulse width of each of IGBT elements, which serves as a line-to-line voltage pulse width of the inverter and limiting a maximum value of the firing pulse width to be equal to or smaller than a predetermined value and a minimum value of the firing pulse width to be equal to or larger than a predetermined value. The first patent document: U.S. Pat. No. 5,671,130; and the second patent document: U.S. Pat. No. 5,990,658.
Also, for example, third and fourth patent documents disclose a technology of monitoring each phase-voltage instruction value input to the PWM controller and limiting a maximum value of each phase-voltage instruction value to be equal to or smaller than a predetermined value and a minimum value of each phase voltage to be equal to or larger than a predetermined value. The third patent document: U.S. Pat. No. 5,912,813; and the fourth patent document: U.S. Pat. No. 6,014,497.
However, the firing pulse width or the voltage instruction value varies for each phase. Therefore, the firing pulse width or the voltage instruction value is required to be limited individually for each phase. That is, to suppress a surge voltage exceeding twice the direct-current bus voltage Vdc by applying the technologies disclosed in the patent documents, if the firing pulse width of each IGBT element or the maximum and minimum values of each phase-voltage instruction value are limited, a plurality of controlling units that control the maximum and minimum values of each phase are required.
Also, one problem of this configuration is that, when the firing pulse width or the voltage instruction value of one phase is limited, an influence on other phases cannot be considered. Moreover, in relation to this problem, there is another problem in which all phases cannot be collectively handled for optimal limitation.
The present invention is devised in view of the above, and an object of the present invention is to provide a power-converter control apparatus, the device being capable of collectively handling all phases and optimally suppressing a surge voltage exceeding twice a direct-current bus voltage.